The present invention relates generally to the processing of photosensitive printing elements such as lithographic graphic arts plates. More particularly, the invention relates to a novel and highly efficacious desensitizing solution, and process for using the solution, to stabilize and protect diazo-sensitized elements in order to prevent the scumming, blinding and grease smudging phenomena commonly encountered with their use in lithographic printing processes.
Lithographic printing utilizes the immiscibility between oleophilic inks and an aqueous dampening fluid on a substantially planar printing plate surface. An oleophilic image area that corresponds to an image to be printed is formed on a plate and the remainder of the plate surface, the non-image area, is or is made hydrophilic in character. The image area accepts greasy ink and transfers the ink during printing; the non-image area is kept damp with water or an aqueous dampening fluid and repels the ink so that no printing occurs from that area. To form such a printing plate, a flat base surface is coated with a very thin layer of a light-sensitive material and exposed to light through a transparent film having opaque areas. A negative transparency of the image desired to be reproduced is used for exposing a so-called negative-acting plate, and a positive transparency is used for exposing a so-called positive-acting plate. Light passes through the clear areas of the negative transparency, which correspond to the image, and causes a reaction in the light-sensitive coating on the underlying plate that "hardens" the coating in the image area. Light does not pass through the opaque areas of the transparency, however, so that the light-sensitive coating on the plate underlying such areas remains unaffected. The plate is then developed by removing the coating from the plate in unexposed areas, which are hydrophilic, or are then made hydrophilic. The positive-acting plate differs from the negative-acting plate in that in the former the light passing through the clear areas of the positive transparency causes the light-sensitive coating on the underlying plate to decompose to some extent, thereby resulting in a solubility differential between image and non-image areas. The exposed areas of the positive-acting plate are removed. In either case, an oleophilic image area is formed on a plate having hydrophilic background areas.
It has also been known for some time that post development treatments of the printing plate may enhance the hydrophilicity of the background areas. Where diazo-sensitized plates have been used, the desensitizing solutions have generally comprised combinations of acids, gums and salts. The acids are employed to etch past any photosensitive or oleophilic residues left on the substrate, leaving a fresh surface with which the gums will react. These gums, generally very high molecular weight compositions, both physically and chemically adhere to the substrate. A variety of salts have been employed to enhance or aid the etching and/or gumming processes. Oftentimes a second gumming operation is performed to assure complete film formation over the background areas.
While these prior art desensitizing compositions and processes have met with considerable success, there are several disadvantages associated with their use which have limited their usefullness and acceptance in the industry and which necessitate a compromise of other desirable lithographic printing parameters. For example, most prior art desensitizing processes must be carefully controlled in order to assure complete desensitization in the background areas without causing "blinding", a condition where the gumming agent reduces the ink receptivity of the image areas. In addition, the acids used pose safety and environmental problems and tend to undercut image areas, thereby reducing the run time for a plate.
Another problem associated with prior art desensitizing systems, and one not previously recognized in the art, is the phenomena of "photo-blinding" which is related in some respects to the known phenomena of "photo-scumming". Photo-scumming is well known in the art. When a lithographic plate is incompletely desensitized and then re-exposed to actinic light, the background is no longer hydrophilic to the degree it was before re-exposing, thereby causing the plate to scum. Thus, the term "photo-scumming". On the other hand, a plate considered properly desensitized with conventional systems containing gums may often show decreased oleophilicity in image areas when re-exposed. Thus, even if the background area remains sufficiently hydrophilic on re-exposure the image may still "photo-blind". It is postulated that non-deactivated diazo in the image area reacts with the gum materials in a tanning or insolubilizing reaction, thereby yielding a slightly hydrophilic substance. Removing the gum from the formulation solves the problem on the image but renders the background sensitive. Removing the gumming agent from the desensitizer formulation eliminates photo-blinding on the image but, of course, results in incomplete desensitization in the background areas. Therefore, with conventional desensitizing systems prevention of photo-scumming causes photo-blinding and vice-versa.
Finally, even through the prior art desensitizers work reasonably well with plates having conventional aluminum substrates, their effectiveness is reduced when used on plates having an anodized aluminum substrate. Thus, scumming in the background areas is more predominant when anodized aluminum substrates are employed, even after a post development treatment with conventional gum desensitizers.